Types of strain

[hkular_2L]

How do you you identify if there is steric, or torsional strain in a molecule and what's the difference between the two?

[Marvin Lu 1E]

Steric strain is when molecules are too close together and torsional strain is when molecules are eclipsed.

[vanessachin2h]

There is also bond angle strain, which is when a molecule's bond angels are not ideal.

[Annie Qing 2F]

Maybe a little more in detail,

Torsional strain is the repulsion between electrons of molecules (methyl groups, for example) on neighboring carbons. When you rotate a molecule around a bond (hence torsional, like torque, which is the force to rotate or twist an object), this is the repulsion when groups pass each other in this rotation.
Steric strain is the repulsion between electrons of molecules on non-neighboring carbons (relatively far away from each other). This repulsion is due to Van der Waals forces.

Torsional strain can be easily identified with the help of a Newman projection. Eclipsed conformations have significantly more torsional strain than staggered ones.
Steric strain is a little harder to see without a model, but when groups are closer than their radii should normally allow (especially common in cis formations), there's a significant amount of steric strain.

Hope this helps!